Fair Value Measurements - Schedule of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Oct. 31, 2019	Jan. 31, 2019
Financial Assets:
Cash equivalents	$ 99,494	$ 78
Marketable securities	580,526	112,777
Commercial paper
Financial Assets:
Marketable securities	48,841	1,243
Corporate bonds
Financial Assets:
Marketable securities		53,214
Agency bonds
Financial Assets:
Marketable securities	78,891	32,604
Corporate and other debt securities
Financial Assets:
Marketable securities	296,979
U.S. government agency securities
Financial Assets:
Marketable securities	106,365	24,017
Fair Value, Recurring Basis
Financial Assets:
Cash equivalents	99,494	78
Marketable securities	580,526	112,777
Total financial assets	682,429	115,199
Financial Liabilities:
Convertible promissory notes – derivative liabilities		163
Other liabilities, noncurrent		163
Total financial liabilities		163
Fair Value, Recurring Basis | Money market funds
Financial Assets:
Cash equivalents	99,494	78
Fair Value, Recurring Basis | Commercial paper
Financial Assets:
Marketable securities	48,841	1,243
Fair Value, Recurring Basis | Corporate bonds
Financial Assets:
Marketable securities		53,214
Fair Value, Recurring Basis | Agency bonds
Financial Assets:
Marketable securities	78,891	32,604
Fair Value, Recurring Basis | Corporate and other debt securities
Financial Assets:
Marketable securities	296,979
Fair Value, Recurring Basis | U.S. government agency securities
Financial Assets:
Marketable securities	106,365	24,017
Fair Value, Recurring Basis | Treasury bills
Financial Assets:
Marketable securities	49,450	1,699
Fair Value, Recurring Basis | Certificate of deposit included in prepaid expenses and other current assets
Financial Assets:
Prepaid expenses and other current assets	100	200
Other assets, noncurrent	2,309	2,144
Fair Value, Recurring Basis | Fair Value, Inputs, Level 1
Financial Assets:
Cash equivalents	99,494	78
Marketable securities	0	0
Total financial assets	99,494	78
Financial Liabilities:
Convertible promissory notes – derivative liabilities		0
Other liabilities, noncurrent		0
Total financial liabilities		0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 1 | Money market funds
Financial Assets:
Cash equivalents	99,494	78
Fair Value, Recurring Basis | Fair Value, Inputs, Level 1 | Commercial paper
Financial Assets:
Marketable securities	0	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 1 | Corporate bonds
Financial Assets:
Marketable securities		0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 1 | Agency bonds
Financial Assets:
Marketable securities	0	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 1 | Corporate and other debt securities
Financial Assets:
Marketable securities	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 1 | U.S. government agency securities
Financial Assets:
Marketable securities	0	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 1 | Treasury bills
Financial Assets:
Marketable securities	0	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 1 | Certificate of deposit included in prepaid expenses and other current assets
Financial Assets:
Prepaid expenses and other current assets	0	0
Other assets, noncurrent	0	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 2
Financial Assets:
Cash equivalents	0	0
Marketable securities	580,526	112,777
Total financial assets	582,935	115,121
Financial Liabilities:
Convertible promissory notes – derivative liabilities		0
Other liabilities, noncurrent		0
Total financial liabilities		0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 2 | Money market funds
Financial Assets:
Cash equivalents	0	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 2 | Commercial paper
Financial Assets:
Marketable securities	48,841	1,243
Fair Value, Recurring Basis | Fair Value, Inputs, Level 2 | Corporate bonds
Financial Assets:
Marketable securities		53,214
Fair Value, Recurring Basis | Fair Value, Inputs, Level 2 | Agency bonds
Financial Assets:
Marketable securities	78,891	32,604
Fair Value, Recurring Basis | Fair Value, Inputs, Level 2 | Corporate and other debt securities
Financial Assets:
Marketable securities	296,979
Fair Value, Recurring Basis | Fair Value, Inputs, Level 2 | U.S. government agency securities
Financial Assets:
Marketable securities	106,365	24,017
Fair Value, Recurring Basis | Fair Value, Inputs, Level 2 | Treasury bills
Financial Assets:
Marketable securities	49,450	1,699
Fair Value, Recurring Basis | Fair Value, Inputs, Level 2 | Certificate of deposit included in prepaid expenses and other current assets
Financial Assets:
Prepaid expenses and other current assets	100	200
Other assets, noncurrent	2,309	2,144
Fair Value, Recurring Basis | Fair Value, Inputs, Level 3
Financial Assets:
Cash equivalents	0	0
Marketable securities	0	0
Total financial assets	0	0
Financial Liabilities:
Convertible promissory notes – derivative liabilities		163
Other liabilities, noncurrent		163
Total financial liabilities		163
Fair Value, Recurring Basis | Fair Value, Inputs, Level 3 | Money market funds
Financial Assets:
Cash equivalents	0	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 3 | Commercial paper
Financial Assets:
Marketable securities	0	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 3 | Corporate bonds
Financial Assets:
Marketable securities		0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 3 | Agency bonds
Financial Assets:
Marketable securities	0	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 3 | Corporate and other debt securities
Financial Assets:
Marketable securities	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 3 | U.S. government agency securities
Financial Assets:
Marketable securities	0	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 3 | Treasury bills
Financial Assets:
Marketable securities	0	0
Fair Value, Recurring Basis | Fair Value, Inputs, Level 3 | Certificate of deposit included in prepaid expenses and other current assets
Financial Assets:
Prepaid expenses and other current assets	0	0
Other assets, noncurrent	$ 0	$ 0